Medical marijuana laws generate significant policy debates regarding drug policy. In particular, if marijuana is a complement or a gateway drug to hard drugs, these laws would increase not only the usage of marijuana but hard drugs such as cocaine and heroin. In this paper, I empirically study the relationships between marijuana and cocaine or heroin by analyzing data on drug possession arrests and treatment admissions. I find that medical marijuana laws increase marijuana usage by 10-20%. However, there is no evidence that cocaine and heroin usage increases after the passage of medical marijuana laws. In fact, the estimates on cocaine and heroin arrests or treatments are uniformly negative. From the arrest data, the estimates indicate a 0-20% decrease in possession arrests for cocaine and heroin combined. From the treatment data, the estimates show a 20% decrease in heroin treatments while no significant effect on cocaine treatments. These results suggest that marijuana could be a substitute for heroin.